Accurate determination of the concentration of water vapor in the exhaust-air of a drier, for instance a tobacco airflow drier, is essential for several reasons. The water vapor concentration in many cases will strongly effect the chemical and physical properties of the material being dried and therefore it must be accurately regulated by appropriately regulating the volume of exhaust or intake air and/or the supply of additional vapor. Furthermore, the economy of an air drying process is primarily determined by the ratio of hot air to vapor and, for a constant mass flow, the terminal humidity of the material being dried can be regulated by a heater if the water vapor concentration of the exhaust air is known.
German Offenlegungsschrift No. 3 114 712 shows a tobacco airflow drier where the value of the concentration of water vapor in the exhaust air is ascertained by an Assmann aspiration-psychometer, termed therein as a "wet-ball measuring instrument". This instrument, however, does not allow a highly accurate determination of the condensing gases or vapors and moreover it is quite sensitive to soiling by dust or condensates formed in this instrument. Also the response time of this type of measuring instrument is relatively long.
A further type of instrument for measuring vapor-content is shown in German Auglegeschrift No. 2 846 826, in which a part of the substance being tested is placed in a measuring tube. Then the temperature, pressure and gas volume of the contents of the measuring tube, after settling for a time, is determined. This is a discontinuous process, however, unsuited to the necessity of continuously regulating a drying process.
Lastly an apparatus of this type for determining the concentrations of condensing as well as non-condensing gases or vapors in a process-gas flow is known from an article entitled "Direct Measurement of the Vapor-Air Content in Drying Chambers" [DIREKTE MESSUNG DES DAMPF-LUFT-GEHALTES IN TROCKENKAMMERN], published in Melliand, 33, 10/1952 at page 967. This includes a gas-evacuation conduit connected to the process gas flow, a gas condenser in the gas evacuation conduit, a continuously operating gas volumeter connected to the gas discharge of the gas condenser, a device to withdraw a sample of test gas from the process gas flow and to move this sample of test gas through the gas condenser and the gas volumeter, and a device to ascertain the concentrations in the measured amount of gas. The sample of test gas is cooled in the gas condenser to about 20.degree. C. and the condensate above the dew point contained therein is removed and quantitatively measured in a graduated measuring glass. The residual air saturated with water vapor passes through a gas flowmeter acting as a gas volumeter. Therefore, on the basis of the temperature measured before and after the gas flowmeter, it is possible to determine the amount of gas and condensate.
This vapor-content test instrument suffers from the drawback that it is relatively expensive because the temperature and the pressure must also be determined in addition to the amounts of gas and condensates. The related computations are laborious and the measurement is discontinuous. Accordingly this instrument is unsuited to regulate the parameters of a continuous drying process.